Epidemiological studies, clinical trials (patients with myocardial infarction), and animal studies have demonstrated that fish consumption, or diets enriched with fish oil (omega-3 polyunsaturated fatty acids, n-3 PUFAs) supplements, significantly reduced the incidence of sudden cardiac death. However, the mechanisms underlying the relationship between n-3 PUFAs and a reduced risk for cardiac arrhythmias remain to be determined. Therefore, we propose to investigate the safety and efficacy of n-3 PUFAs using a well-established canine model of lethal ventricular arrhythmias. The mechanisms altering cardiac electrophysiology will be examined in cardiac myocytes isolated from the hearts of these animals. The central hypothesis is that chronic treatment with n-3 PUFAs attenuates the electrophysiological abnormalities contributing to sudden cardiac death in a post-myocardial infarction canine model of sudden cardiac death. The following specific aims will be used to test this hypothesis: Specific Aim #1 will test the hypothesis that the chronic ingestion of n-3 PUFA supplements will prevent ischemia-induced tachyarrhythmias (by preventing re-entrant and/or triggered arrhythmias). Specific Aims #2 and #3 will investigate the effects of the chronic administration of n-3 PUFAs on the electrophysiology and calcium regulation of cardiomyocytes prepared from the hearts of the dogs, respectively. In particular, Specific Aim 2 will test the following hypotheses: n-3 PUFAs will a) normalize the sarcolemmal ion currents improving the ventricular myocyte action potential and b) reduce both early and delayed afterdepolarizations in dogs susceptible to ventricular fibrillation. Finally, Specific Aim #3 will test the following hypotheses: a) n-3 PUFAs will block L-type calcium current reducing calcium entry and thereby preventing calcium overload-induced spontaneous calcium release in dogs susceptible to ventricular fibrillation;b) n-3 PUFAs will alter ryanodine receptor function, restoring a more normal sarcoplasmic reticular calcium release in dogs susceptible to ventricular fibrillation;and c) n-3 PUFAs will alter the sarcoplasmic reticular (SR) calcium load, restoring a more normal SR calcium load in the dogs susceptible to ventricular fibrillation. Lay summary: This project will examine the safety of fish oil supplements after a heart attack. This project will also determine the effectiveness of fish oil supplements in reducing sudden death due to a lethal abnormal heart rhythm after a heart attack.